Common Plastic Injection Molding Problems (and How to Solve Them)

Plastic injection molded parts

Plastic injection molding is a versatile and efficient way to produce durable, detailed parts of endless sizes, shapes, and purposes. However, since the process is both an art and a science it is not without its challenges.

This article will explore eight of the most common plastic injection molding problems, including what causes them and what you can do to avoid them.

1. Flow lines

What are they? Flow lines are off-color patterns, rings, streaks, or lines that appear on a component. The lines usually don’t affect the part’s integrity, but their appearance can be unacceptable, especially in consumer products.

What causes them? Flow lines typically are caused by varying flow rates as the molten plastic resin moves through the mold or as changes occur in the thickness of the mold’s walls. A slow injection speed can cause the plastic to harden at different rates, which can lead to discoloration.

How can you prevent them? Here are some steps to take to mitigate flow lines:

  • Increase the injection speed and temperature so that the plastic fills the mold before it cools
  • Round areas where thickness varies to help keep flow rate consistent
  • Relocate mold gates to create more distance between them and the mold coolant

2. Burn marks

What are they? Burn marks are black or rusty discolorations that appear on the surface or edge of a molded plastic part. Most of the time, these marks affect only the appearance of the part, not its integrity. However, extensive burn marks can be a sign of degradation.

What causes them? Excessive heating or too fast injection speeds are common causes. Other suspects are trapped air that has overheated inside the part.

How can you prevent them? Here are steps to decrease or eliminate burn marks:

  • Lower injection speeds
  • Reduce the melt and mold temperature
  • Shorten the mold cycle time
  • Enlarge gas vents and gates to allow trapped air to escape

3. Warping

 What is it? Warping is the unintentional twisting, bending, shrinking, or other type of deformation that can occur in a plastic injection molded part.

What causes it? Uneven cooling rates inside the mold are the likely cause of warping.

How can you prevent it? Try these fixes for warping prevention:

  • Make sure your cooling process is gradual in terms of both time and speed
  • Decrease the temperature of the material or mold
  • Use a material that is less prone to shrinkage
  • Ensure the mold has a uniform wall thickness

4. Air pockets

 What are they? Also called vacuum voids, air pockets are areas in the molded part that have trapped air inside them or close to the surface.

What causes them? Air pockets can happen when there is not enough holding pressure to force trapped air out of the mold cavity. Other possible causes are misalignment of the mold halves and uneven curing.

How can you prevent them? Here are ways to avoid or minimize air pockets:

  • Increase the injection pressure to force out trapped air
  • Use a material with a lower viscosity rate
  • Position gates close to the thickest parts of the mold

5. Sink marks

What are they? Sink marks are unintentional depressions on the surface of a molded part.

What causes them? Sink marks can occur when the inner part of a molded component shrinks, pulling the material inward. The cooling may be happening too slowly, or the plastic may not be cooling down enough while it is inside the mold. Another factor may be the pressure inside the cavity or too much heat at the gate.

How can you prevent them? Try these steps to reduce sink marks:

  • Decrease the mold temperature
  • Raise the holding pressure and time
  • Allow the plastic more time to cool and cure inside the mold
  • Use thinner walls to allow for faster cooling

 6. Weld lines

What are they? A weld line is a visible seam where two areas of the molten plastic meet.

What causes them? The primary cause is weak bondage between two or more flow fronts. Weld lines can lower the integrity of the part.

How can you prevent them? Here’s how to decrease the chances of weld lines:

  • Increase the temperature of the mold or the molten plastic
  • Raise the injection speed
  • Use a less viscous plastic
  • Change the flow pattern to a single-source flow
  • Redesign the mold to eliminate the problem area

7. Short shot

 What is it? “Short shot” is the term used for a partially-filled mold cavity. The incomplete part may not be able to function correctly or have diminished integrity or appearance.

What causes it? Short shots usually are caused by a restricted flow of resin. Sometimes the gate is blocked or too narrow. Or, trapped air pockets may be hindering the flow. Another possibility is inadequate injection pressure.

How can you prevent it? Here are steps to take to get rid of short shots:

  • Use a less viscous plastic
  • Increase the mold temperature
  • Alter the mold design to include wider channels
  • Add or enlarge air vents to allow trapped air to escape

8. Flash

What is it? Flash occurs when some molten plastic flows outside the intended flow channels and into spaces between the tooling plates or around the injector pin.

What causes it? A poor mold design or a degraded mold is the most common cause of flash. Clamps also may not be tight enough, or the injection pressure may be too high, forcing molten plastic out of the mold.

How can you prevent it? Here are ways to fix flash:

  • Increase your clamp pressure
  • Adjust the injection speed or pressure
  • Retool or redesign the mold
  • Replace a degraded mold

Now that you understand some of the typical problems that can occur with plastic injection molding, you can take steps to prevent them. Many of them can be solved with reasonably easy remedies. However, keep in mind that the sooner you attend to them, the better.

At ASPM, we showcase our established history of quality tooling and molding by implementing our rigorous approach to plastic product development. We offer complete support from product design and feasibility, through routine injection mold maintenance and repair. Our highly trained staff is prepared to handle plastic projects with varying complexity in our robust 24-hour facility. Contact us today to discuss your next project.